Vagal nerve stimulation has been applied to modulate various physiologic functions and treat various diseases. One example is the modulation of cardiac functions in a patient suffering heart failure or myocardial infarction. The myocardium is innervated with sympathetic and parasympathetic nerves including the cardiac branches of the vagus nerve. Activities in the vagus nerve, including artificially applied electrical stimuli, modulate the heart rate and contractility (strength of the myocardial contractions). Electrical stimulation applied to the vagus nerve is known to decrease the heart rate and the contractility, lengthening the systolic phase of a cardiac cycle, and shortening the diastolic phase of the cardiac cycle. This ability of the vagal nerve stimulation is utilized, for example, to control myocardial remodeling.
In addition to treating cardiac disorders such as myocardial remodeling, vagal nerve stimulation is also know to be effective in treating disorders including, but not limited to, depression, anorexia nervosa/eating disorders, pancreatic function, epilepsy, hypertension, inflammatory disease, and diabetes. The intended therapy outcomes of vagal nerve stimulation in treating such disorders may be difficult to measure, either acutely or chronically, for purposes of therapy titration or optimization. Therefore, there is a need for titrating or optimizing vagal nerve stimulation using parameters other than the intended therapy outcomes.